1. Field of the Invention
The invention is related to the field of communication systems, and in particular, to systems and methods of providing distributed and discrete amplification of optical signals.
2. Statement of the Problem
Many communication companies use fiber optic cabling as a media for transmitting data because of its high-bandwidth capacity. Fiber optic cables reliably transport optical signals over long distances. Over a distance, optical signals attenuate in the fiber due to Rayleigh scattering. The attenuation may be recovered by an optical amplifier. However, the optical amplifier adds noise to the optical signals. The noise accumulation on the optical signals can especially be a problem for ultra long haul transmissions.
Optical amplifiers may be discrete amplifiers or distributed amplifiers. Distributed amplifiers use the transmission fiber carrying the optical signals as a gain medium. Discrete amplifiers do not use transmission fiber as a gain medium, but use another type of fiber or component as the gain medium.
One type of discrete amplifier is an Erbium-Doped Fiber Amplifier (EDFA). In an EDFA, an Erbium-doped fiber receives optical signals from a transmission fiber. A pump transmits light having a wavelength of 980 nm onto Erbium-doped fiber concurrently as the optical signals travel over the Erbium-doped fiber. The properties of the Erbium-doped fiber act to absorb the pumped light and generate a gain in the optical signals using the absorbed light. The Erbium-doped fiber acts as the gain medium, not the transmission fiber. Unfortunately, traditional EDFA's have a limit on the gain bandwidth they can produce. For instance, a C-band EDFA has a gain bandwidth of about 30 to 40 nm (1530 nm to 1570 nm). As the demand for capacity increases, the C-band may not be enough to handle the needed capacity.
There are certain types of discrete amplifiers that have a wider gain bandwidth than a traditional EDFA. In one type of discrete amplifier, a splitter separates optical signals into three bands: the C-band, the L-band, and the S-band. The C-band comprises a range of wavelengths of approximately 1530 nm to 1570 nm. The L-band comprises a range of wavelengths of approximately 1570 nm to 1600 nm. The S-band comprises a range of wavelengths of approximately 1500 nm to 1530 nm. The splitter transfers the three different bands to three different rare earth doped fiber amplifiers. Each amplifier is configured to amplify one of the bands. A combiner receives the amplified bands and re-combines the optical signals. This configuration generates a gain bandwidth of about 100 nm. Unfortunately, the splitter, the three amplifiers, and the combiner can be expensive and complicated to implement. This configuration is discussed further below and is shown in FIG. 1a. 
Another discrete amplifier having a wider gain bandwidth than a traditional EDFA is a Telluride-based EDFA (T-EDFA). Telluride-based EDFAs have a gain bandwidth of about 75 nm. Unfortunately, fiber non-linearity may be a problem with Telluride-based EDFAs. The fiber non-linearity may cause some of the 75 nm gain bandwidth to be unusable.
Another discrete amplifier with a wider gain bandwidth than a traditional EDFA is a fluoride-based EDFA (F-EDFA). F-EDFAs have a gain bandwidth of about 100 nm. Unfortunately, the F-EDFAs may generate an undesirable noise figure, especially for the longer and shorter wavelength bands of the 100 nm gain bandwidth. A gain region having a high noise figure may have a significantly reduced transmission distance compared to a gain region having a low noise figure. F-EDFAs are discussed further below and shown in FIG. 2a. 
Another type of discrete amplifier with a wider gain bandwidth than a traditional EDFA is a Raman amplifier. In a discrete Raman amplifier, a fiber span within the Raman amplifier receives optical signals from a transmission fiber. The fiber span may be a highly doped fiber, such as a dispersion compensating fiber. A Raman pump backward pumps light onto the fiber span carrying the optical signals. Based on the “Raman Effect”, the light amplifies the optical signals traveling on the fiber span. The discrete Raman amplifier provides a wider gain bandwidth than traditional EDFAs and allows for replacement of high-powered EDFAs. However, the discrete Raman amplifier generates a higher noise figure than EDFAs and high power Raman pumps are needed to generate the wide gain bandwidth.
Distributed amplifiers have been used with discrete amplifiers to provide a higher gain to optical signals. For the distributed amplifier, a Raman pump pumps light onto a transmission fiber span. The light amplifies optical signals traveling over the fiber span. A discrete amplifier then receives the optical signals and amplifies the optical signals. The distributed amplifier and the discrete amplifier work together to amplify the optical signals. One problem with the current distributed/discrete amplifier configurations is that the distributed amplifier only amplifies the same wavelengths as the discrete amplifier. Thus, this configuration may still have the problems caused by the discrete amplifiers' limited gain bandwidth and/or high pump powers. The distributed/discrete amplifier configuration is discussed further below and shown in FIG. 3a. 